Find the shared library denoted by library (a string) and link it
into the running Guile application. When everything works out, return a
Scheme object suitable for representing the linked object file.
Otherwise an error is thrown. How object files are searched is system
dependent.

Normally, library is just the name of some shared library file
that will be searched for in the places where shared libraries usually
reside, such as in /usr/lib and /usr/local/lib.

Unlink the indicated object file from the application. The
argument dobj must have been obtained by a call to
dynamic-link. After dynamic-unlink has been
called on dobj, its content is no longer accessible.

Search the dynamic object dobj for the C function
indicated by the string name and return some Scheme
handle that can later be used with dynamic-call to
actually call the function.

Regardless whether your C compiler prepends an underscore ‘_’ to
the global names in a program, you should not include this
underscore in function. Guile knows whether the underscore is
needed or not and will add it when necessary.

Call the C function indicated by func and dobj.
The function is passed no arguments and its return value is
ignored. When function is something returned by
dynamic-func, call that function and ignore dobj.
When func is a string , look it up in dynobj; this
is equivalent to

(dynamic-call (dynamic-func funcdobj) #f)

Interrupts are deferred while the C function is executing (with
SCM_DEFER_INTS/SCM_ALLOW_INTS).

Call the C function indicated by func and dobj,
just like dynamic-call, but pass it some arguments and
return its return value. The C function is expected to take
two arguments and return an int, just like main:

int c_func (int argc, char **argv);

The parameter args must be a list of strings and is
converted into an array of char *. The array is passed
in argv and its size in argc. The return value is
converted to a Scheme number and returned from the call to
dynamic-args-call.

When dynamic linking is disabled or not supported on your system,
the above functions throw errors, but they are still available.

As you can see, after calling dynamic-unlink on a dynamically
linked library, it is marked as ‘(unlinked)’ and you are no longer
able to use it with dynamic-call, etc. Whether the library is
really removed from you program is system-dependent and will generally
not happen when some other parts of your program still use it. In the
example above, libc is almost certainly not removed from your
program because it is badly needed by almost everything.

The functions to call a function from a dynamically linked library,
dynamic-call and dynamic-args-call, are not very powerful.
They are mostly intended to be used for calling specially written
initialization functions that will then add new primitives to Guile.
For example, we do not expect that you will dynamically link
libX11 with dynamic-link and then construct a beautiful
graphical user interface just by using dynamic-call and
dynamic-args-call. Instead, the usual way would be to write a
special Guile<->X11 glue library that has intimate knowledge about both
Guile and X11 and does whatever is necessary to make them inter-operate
smoothly. This glue library could then be dynamically linked into a
vanilla Guile interpreter and activated by calling its initialization
function. That function would add all the new types and primitives to
the Guile interpreter that it has to offer.

From this setup the next logical step is to integrate these glue
libraries into the module system of Guile so that you can load new
primitives into a running system just as you can load new Scheme code.

There is, however, another possibility to get a more thorough access to
the functions contained in a dynamically linked library. Anthony Green
has written libffi, a library that implements a foreign
function interface for a number of different platforms. With it, you
can extend the Spartan functionality of dynamic-call and
dynamic-args-call considerably. There is glue code available in
the Guile contrib archive to make libffi accessible from Guile.